1. Field of the Invention
The present invention relates to a composition for recovery of petroleum and gas (hereinafter referred to as petroleum, etc.) present in oil reservoir or gas reservoir (hereinafter referred to as oil reservoir, etc.), as well as to a method for recovery of petroleum, etc. using the composition.
2. Description of the Related Art
For recovery of petroleum from underground oil reservoir, there has long been used generally a so-called primary recovery method which comprises finding an oil reservoir by exploration, constructing a drilling facility above the oil reservoir, and producing petroleum as long as the production is profitable.
In recent years, as the technique for exploration of oil reservoir advanced and the conditions of oil reservoir came to be known more precisely, there have been used secondary and tertiary recovery methods which comprise injecting a fluid into an underground oil reservoir, in which the production rate is already low as the result of primary recovery, to push the remaining petroleum to one side, thereby producing it for a higher recovery ratio. A typical example of the secondary recovery method is water flooding and there are, as the tertiary method, carbon dioxide flooding, steam flooding, micellar polymer flooding, etc.
Oil reservoirs differ in conditions, for example, permeability, oil viscosity, temperature, etc. Some oil reservoirs comprise a plurality of layers of different permeabilities. In such a case, with the above approach, the injected fluid mostly permeates the high-permeability portion of reservoir; as a result, the petroleum in reservoir is not replaced sufficiently; thus, efficient recovery of petroleum has been impossible.
Meanwhile, in the recovery of gas from an underground gas reservoir, the gas pressure in the vicinity of well decreases with the production of gas, causing a water-coning phenomenon; or the formation water which has permeated the high-permeability portion of the gas reservoir, is taken out together with gas, in some cases. This has resulted in low gas productivity and moreover has required a high cost for water disposal.
To prevent such low productivity of petroleum, etc., there has recently been used often a method which comprises injecting a polymer and a crosslinking agent into the high-permeability portion of an oil reservoir to form a polymer gel.
The above method includes, for example, a method using a gelling system of an acrylamide type polymer and a crosslinking metal ion to suppress the water permeability of the high-permeability portion of the oil reservoir (U.S. Pat. No. 3,762,476). In this method, an aqueous solution of an acrylamide type polymer and a crosslinking agent solution are alternately injected into the Oil reservoir to allow the two solutions to give rise to gelling in the oil reservoir; however, since the mixing of the aqueous polymer solution and the crosslinking agent solution in the oil reservoir is insufficient, no strong gel is formed.
To solve this problem, there was developed a method of injecting a polymer solution and a crosslinking agent simultaneously. There were proposed, for example, a gelling system using an acrylamide type polymer and chromium acetate (U.S. Pat. No. 4,683,949) and a gelling system using an acrylamide type polymer and a phenol type compound/formaldehyde (U.S. Pat. No. 4,246,124).
The above approaches are effective for oil reservoirs, etc. having a temperature of about 70.degree. C. or lower; however, when they are applied to oil reservoirs, etc. having a temperature higher than 70.degree. C. and containing, particularly in the water present therein, bivalent metal ions (e.g. calcium ion and magnesium ion) in an amount of about 100 ppm or more, the formed polymer gel is destroyed or the gel causes syneresis, resulting in insufficient recovery.
In treating an underground oil reservoir, etc. of high permeability with a polymer gel, it is necessary to form a polymer gel of several cubic meters to several hundreds of cubic meters per well, in the oil reservoir, etc. and, in order to enable the formation of said gel, fluid(s) must be injected into the reservoir, etc. in several hours to ten-odd days, although these conditions differ depending upon the properties of the reservoir, etc. To prevent gelling during the injection, it is necessary to make the gelling time longer than the injection time; however, when the gelling time is longer than necessary, insufficient gelling takes place because the uncrosslinked gel permeates even the low-permeability portion of oil reservoir or because the gelling composition is diluted by the oil-reservoir water. Accordingly, in order to treat an oil reservoir, etc. of high permeability with a polymer gel to recover petroleum, etc. effectively, it is requisite to determine the amount, rate, etc. of gelling composition injected and select a gelling composition comprising a polymer and a crosslinking agent, having an appropriate gelling time.